We demonstrate for 24 metal oxide (MOx) nanoparticles that it is possible to use conduction band energy levels to delineate their toxicological potential at cellular and whole animal levels. Among the materials, the overlap of conduction band energy (Ec) levels with the cellular redox potential (−4.12 to −4.84 eV) was strongly correlated to the ability of Co3O4, Cr2O3, Ni2O3, Mn2O3 and CoO nanoparticles to induce oxygen radicals, oxidative stress and inflammation. This outcome is premised on permissible electron transfers from the biological redox couples that maintain the cellular redox equilibrium to the conduction band of the semiconductor particles. Both single parameter cytotoxic as well as multi-parameter oxidative stress assays in cells showed excellent correlation to the generation of acute neutrophilic inflammation and cytokine responses in the lungs of CB57 Bl/6 mice. Co3O4, Ni2O3, Mn2O3 and CoO nanoparticles could also oxidize cytochrome c as a representative redox couple involved in redox homeostasis. While CuO and ZnO generated oxidative stress and acute pulmonary inflammation that is not predicted by Ec levels, the adverse biological effects of these materials could be explained by their solubility, as demonstrated by ICP-MS analysis. Taken together, these results demonstrate, for the first time, that it is possible to predict the toxicity of a large series of MOx nanoparticles in the lung premised on semiconductor properties and an integrated in vitro/in vivo hazard ranking model premised on oxidative stress. This establishes a robust platform for modeling of MOx structure-activity relationships based on band gap energy levels and particle dissolution. This predictive toxicological paradigm is also of considerable importance for regulatory decision-making about this important class of engineered nanomaterials.

Silver nanoparticles (Ag NPs) are commonly added to various consumer products and materials to impair bacterial growth. Recent studies suggested that the primary mechanism of antibacterial action of silver nanoparticles is release of silver ion (Ag(+)) and that particle-specific activity of silver nanoparticles is negligible. Here, we used a genome-wide library of Escherichia coli consisting of ∼4000 single gene deletion mutants to elucidate which physiological pathways are involved in how E. coli responds to different Ag NPs. The nanoparticles studied herein varied in both size and surface charge. AgNO3 was used as a control for soluble silver ions. Within a series of differently sized citrate-coated Ag NPs, smaller size resulted in higher Ag ion dissolution and toxicity. Nanoparticles functionalized with cationic, branched polyethylene imine (BPEI) exhibited equal toxicity with AgNO3. When we used a genome-wide approach to investigate the pathways involved in the response of E. coli to different toxicants, we found that only one of the particles (Ag-cit10) exhibited a pattern of response that was statistically similar to that of silver ion. By contrast, the pathways involved in E. coli response to Ag-BPEI particles were more similar to those observed for another cationic nanoparticle that did not contain Ag. Overall, we found that the pathways involved in bacterial responses to Ag nanoparticles are highly dependent on physicochemical properties of the nanoparticles, particularly the surface characteristics. These results have important implications for the regulation and testing of silver nanoparticles.

Background: Differences in interlaboratory research protocols contribute to the conflicting data in the literature regarding engineered nanomaterial (ENM) bioactivity.Objectives: Grantees of a National Institute of Health Sciences (NIEHS)-funded consortium program performed two phases of in vitro testing with selected ENMs in an effort to identify and minimize sources of variability.Methods: Consortium program participants (CPPs) conducted ENM bioactivity evaluations on zinc oxide (ZnO), three forms of titanium dioxide (TiO2), and three forms of multiwalled carbon nanotubes (MWCNTs). In addition, CPPs performed bioassays using three mammalian cell lines (BEAS-2B, RLE-6TN, and THP-1) selected in order to cover two different species (rat and human), two different lung epithelial cells (alveolar type II and bronchial epithelial cells), and two different cell types (epithelial cells and macrophages). CPPs also measured cytotoxicity in all cell types while measuring inflammasome activation [interleukin-1β (IL-1β) release] using only THP-1 cells.Results: The overall in vitro toxicity profiles of ENM were as follows: ZnO was cytotoxic to all cell types at ≥ 50 μg/mL, but did not induce IL-1β. TiO2 was not cytotoxic except for the nanobelt form, which was cytotoxic and induced significant IL-1β production in THP-1 cells. MWCNTs did not produce cytotoxicity, but stimulated lower levels of IL-1β production in THP-1 cells, with the original MWCNT producing the most IL-1β.Conclusions: The results provide justification for the inclusion of mechanism-linked bioactivity assays along with traditional cytotoxicity assays for in vitro screening. In addition, the results suggest that conducting studies with multiple relevant cell types to avoid false-negative outcomes is critical for accurate evaluation of ENM bioactivity.

Metal oxide nanoparticles (MOx NPs) are used for a host of applications, such as electronics, cosmetics, construction, and medicine, and as a result, the safety of these materials to humans and the environment is of considerable interest. A prior study of 24 MOx NPs in mammalian cells revealed that some of these materials show hazard potential. Here, we report the growth inhibitory effects of the same series of MOx NPs in the bacterium Escherichia coli and show that toxicity trends observed in E. coli parallel those seen previously in mammalian cells. Of the 24 materials studied, only ZnO, CuO, CoO, Mn2O3, Co3O4, Ni2O3, and Cr2O3 were found to exert significant growth inhibitory effects; these effects were found to relate to membrane damage and oxidative stress responses in minimal trophic media. A correlation of the toxicological data with physicochemical parameters of MOx NPs revealed that the probability of a MOx NP being toxic increases as the hydration enthalpy becomes less negative and as the conduction band energy approaches those of biological molecules. These observations are consistent with prior results observed in mammalian cells, revealing that mechanisms of toxicity of MOx NPs are consistent across two very different taxa. These results suggest that studying nanotoxicity in E. coli may help to predict toxicity patterns in higher organisms.